EmonTv V3 voltage output

I have been trying to use my emonTx V3 to trigger an SSR. The 3.3v output is not enough, so I thought I'd just use the 5v output. However, when I take a reading this actually gives about 3v.

The emonTx V3 is powered by the standard 9v AC adaptor from the online shop (jumper JP2 closed). I unplugged all CTs and temperature probes and still I onll get 3v from the 5v terminal.

Is there a problem with my emonTx V3, or is this something that is know to be a problem?

As a test I plugged in a 5v USB input in addition to the AC adaptor. If I do this the 5v terminal block now actually reads 5v. However, it would be nice to be getting the correct voltage without needing an additional power source (as this is one new feature of the V3)

Robert Wall's picture

Re: EmonTv V3 voltage output

As I more-or-less inferred in my PM to you, 3.3 V is the correct voltage for the supply to the processor. I've recently added a paragraph to the Building Blocks page "Sources of error..." where I calculate that the 3.3 V rail should be within approx. 1% of that nominal value. As far as I'm aware, 5 V has never been the normal operating voltage for any of the emonTx range, because that exceeds the voltage rating of the RFM12B radio module.

AllenConquest's picture

Re: EmonTv V3 voltage output

But in the diagram for the emonTx V3 here http://wiki.openenergymonitor.org/index.php?title=File:EmonTx_V3.2_Labelled_Ports.jpg it shows pin 1 on the break-out terminals as having 5v. 

I'm guessing my answer is here http://wiki.openenergymonitor.org/index.php?title=EmonTx_V3#Optical_Pulse_Counting, where is states that pin 1 is 5v if connected to a USB power supply?

Robert Wall's picture

Re: EmonTv V3 voltage output

In the V2, the dc input is regulated down to 3.3 V for the processor and RFM12B. If you use the USB input, the 5 V rail is available on-board but you can't use it directly in conjunction with an output pin of the processor (see Fig. 13.1 of the Atmel data sheet for why, the input protection diodes get in the way) - you need to interpose a transistor driver or similar. The processor has always worked at 3.3 V.

The V3 is pretty much the same, except that two regulators are used to obtain the 3.3 V rail. One is used if the 5 V USB or batteries are the power source, the other if the ac adapter is the power source. There isn't a 5 V rail available if the ac adapter is the power source in use, and I think I'm right in saying there was never any intention to make one available. The schematics on Solderpad tell all.

If you have an external dc supply available - at any voltage - then with the V3 that option of a driver transistor is open to you too. Bear in mind there's a difference between inputs and outputs, you can divide down a voltage to present it to a lower voltage input, or you can feed current and limit it to a safe value. You can't do that when the pin is configured as an output.

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